This is pretty clear from a myriad of complaints from Bestmengqin's blog

FIFA Mobile's "Seasoned Athlete Gullit milestone reported to be incomplete even after unlocking the fix 'Rooney. This is combined with the announcement of Gullit Event comprised of nine milestones to be completed.

After completing all milestones, you are eligible to claim the title of FUT 22 Coins Prime Icon Gullit at any location you choose.

A particular milestone can be extremely difficult for those trying to complete this event. This is pretty clear from a myriad of complaints on EA forums and on social media.The Seasoned Athlete achievement does not appear to be completed even after getting the Seasons Legend player 'Rooney'. The number of restarts, cleaning data from apps, or re-installs appears to have any effect.

This is a huge cause of bother for players, especially for those that have already completed all other eight milestones only to be held back by an incomplete Seasoned Athlete.Some now even believe that Rooney is one of those players that doesn't count towards Gullit Milestones.Fortunately, Rooney does count, and Seasoned Athlete isn't getting marked as complete upon unlocking him as there indeed is a bug in FIFA Mobile.

This is pretty clear from an official statement by a Community Manager on EA forums. According to the moderator, the issue is currently being investigated. If all goes according to Buy FIFA 22 Coins plan, FIFA Mobile Gullit will be addressed either through an update to fix bugs, or through a server-side fix. For now, you can sit back and watch.